Conventionally, in a vehicle control mechanism, a telescopic shaft in which a male shaft and a female shaft are in spline-fitting has been used as a part of a steering mechanism in order to absorb axial direction displacement which is generated during running of a vehicle and not to propagate the displacement or vibration onto a steering wheel. FIG. 1 is a side view illustrating an example thereof. In the figure, reference symbols “a” and “b” denote telescopic shafts. In the telescopic shaft a, a male shaft and a female shaft are in spline-fitting. The telescopic shaft a is required to have an ability of absorbing the axial direction displacement which is generated during running of the vehicle and of not propagating the displacement or the vibration onto the steering wheel. Such an ability is usually required when the vehicle body is in a sub-frame structure in which a portion “c” for fixing an upper part of the steering mechanism and a frame “e” to which a steering rack “d” is fixed are separately provided and both members are fastened and fixed through an elastic member “f” such as rubber interposed therebetween. There is also another case in which a telescopic function is required for an operator, when fastening a steering shaft joint “g” to a pinion shaft “h”, to temporarily contract the telescopic shaft to then fit and fasten it to the pinion shaft “h”. Further, the telescopic shaft b in an upper part of the steering mechanism also has the male shaft and the female shaft being in spline fitting. This telescopic shaft b is required to have a function of extending and contracting in the axial direction since it is required to have a function of moving a steering wheel “I” in the axial direction to adjust the position thereof in order to obtain an optimal position for a driver for driving the vehicle. In all the cases described above, the telescopic shaft is required to reduce rotational looseness noise in a spline portion, to reduce a rotational looseness feeling on the steering wheel and to reduce a sliding resistance during a sliding movement in the axial direction.
On that account, in conventional cases, metallic noise, metallic rattling noise, or the like has been absorbed or reduced and the sliding resistance and the looseness in a rotation direction have been reduced by applying a nylon coating to a spline portion of the male shaft and applying grease to a sliding portion. Further, since a complicated process or advanced finishing is necessary to apply the nylon coating, forming a polytetrafluoroethylene coating which has proper elasticity and is excellent in lubricating property or wearing resistance has been proposed (see Patent Document 1 for reference). Moreover, instead of the nylon coating, a resin sliding sleeve has been interposed between the male shaft and the female shaft to reduce the sliding resistance and to prevent the rotational looseness from occurring (see Patent Documents 2 to 4 for reference). In addition, it has been proposed that a recess portion for storing the grease is formed in a center part of a tooth tip surface or a tooth bottom surface of at least one of the male shaft and the female shaft to facilitate the supply and holding of the grease (see Patent Document 5 for reference), or that at least a circumferential direction end portion of the tooth tip of a female spline tooth is processed into an arc shape and covered with a mold release agent and the telescopic shaft in which a gap between the male shaft and the female shaft is filled with a resin is sealed (see Patent Document 6 for reference).
Patent Document 1: JP-A-2003-54421;
Patent Document 2: JP-B-5-7224;
Patent Document 3: JP-A-11-208484;
Patent Document 4: JP-A-2000-74081;
Patent Document 5: JP-A-2004-245372; and
Patent Document 6: JP-UM-B-7-49073.